The Lensmaker’s Formula
So far, we have treated lenses in terms of their focal lengths.
How do you make a lens with focal length f ?
Start with Snell’s Law. Consider a plano-convex lens:
Snell’s Law at the convex surface:
The bend-angle b is just given by:
The bend-angle b also defines the focal length f:
The angle q can be written in terms of R, the radius of curvature of the lens :
Putting these last equations together,
Look, h and q drop out !!
This holds for any incident parallel ray